This application claims the priority of 198 24 915.2, filed Jun. 4, 1998, the disclosure of which is expressly incorporated by reference herein.
The present invention relates to a method for changing the operating mode of a direct-injection Otto-cycle internal-combustion engine between a stratified charge operation with fuel injection during the compression stroke and a lean mixture formation and an operating mode with a homogeneous mixture formation by fuel injection during the intake stroke of the internal-combustion engine.
More particularly, the present invention relates to an operating mode changing method with lean fuel/air mixture being formed with fuel/air ratios .lambda.&gt;1, and an operating mode with a homogeneous mixture formation provided for higher load ranges of the internal-combustion engine by fuel injection during the intake cycle, comprising the steps of operating the internal-combustion engine, for periodic regeneration of an .sub.Nox storage catalyst for decontaminating exhaust gases of the internal-combustion engine flowing through at lean fuel/air ratios (.lambda.), with the homogeneous mixture formation and rich fuel/air ratios (.lambda.) below the stoichiometric mixture ratio .lambda.=1.
In a direct-injection Otto cycle internal-combustion engine, the combustion can in principle take place in two different operating modes. The difference is basically the quality of the fuel/air mixture formed in the combustion space. In the operating mode with a homogeneous mixture formation, in every operating cycle, an intake air flow assigned to the respectively present load point of the internal-combustion engine is admitted into the combustion space, and intake air flow can be adjusted by a corresponding throttling of the intake air flow rate.
During the intake stroke in such an operating mode, the fuel is injected directly into the injection space and the intake air existing therein. In the remaining time period to the ignition at the end of the compression stroke, a homogeneous mixture is formed with fuel-air ratios in the ignitable range, that is, approximately .lambda.=1. The lambda fuel-air ratio normally indicates the ratio of the fuel quantity to the air quantity of the combustion space charge.
With such a homogeneous mixture formation, the entire combustion space is therefore filled with the ignitable mixture so that it is always ensured that the ignition of the combustion space charge can take place at the electrodes of a spark plug. A quantity control therefore takes place of the load of the internal-combustion engine by way of the throttling of the intake air flow rate and the adjustment of the intake air flow apportioned per operating cycle to a cylinder for the mixture formation.
In the stratified charge operation, the intake air is guided in an unthrottled manner; i.e., the maximally conventional air quantity is taken in. The fuel injection takes place during the compression stroke and therefore at a late point in time in the operating cycle shortly before the ignition. A non-homogeneous stratified fuel/air mixture with locally different fuel/air ratios is formed in the combustion space. That is, a fuel-rich mixture cloud is present close to the injector which can be ignited by the ignition spark. The fuel-rich mixture cloud is surrounded with regions of a high air excess.
In the stratified charge operation, a quality control of the operating load of the internal-combustion takes place by the adjustment of the injection quantity from which locally present ignitable mixture is formed. Relative to the total volume, in the stratified charge operation, a combustion space charge with fuel/air ratios .lambda.&gt;1 is formed, whereby, in the stratified charge operation, advantages can be achieved with respect to the fuel consumption of over 20% in comparison to the homogeneous mixture formation with .lambda.=1 with the same operating load.
At higher operating loads of the internal-combustion engine, however, the advantageous charge stratification can lead to an increasing deterioration of the efficiency. Furthermore, in these power ranges of the internal-combustion engine, mixture preparation problems cannot be avoided with charge stratification. The internal-combustion engine is therefore operated in the lower and medium partial load range by a charge stratification, and the operating mode with a homogeneous mixture formation is provided for the higher load ranges.
The exhaust gas of the internal-combustion engine is decontaminated by an NO.sub.x storage catalyst which, in the stratified charge operation, adsorbs the nitrogen oxides contained in the flowing-through oxygen-rich exhaust gas of the internal-combustion engine. The storage catalyst additionally functions as a three-way catalyst for the operation with a homogeneous mixture formation and stoichiometric or rich fuel/air ratios .lambda..ltoreq.1 in a known manner. The storage catalyst adsorbs the nitrogen oxides emitted in the stratified charge operation and must be periodically regenerated.
For regenerating the storage catalyst, DE 43 15 278 A1 (corresponding to U.S. Pat. No. 5,628,186). suggests that a reducing agent, such as ammonia, be added to the exhaust gas. Together with the admixed reducing agent, the nitrogen oxides contained in the exhaust gas, according to the method of the selective catalytic reduction, are to be converted to environmentally friendly nitrogen and water. The known method provides that, for apportioning the reducing agent, the reducing agent rate charged into the exhaust gas be adjusted as a function of operation-relevant parameters of the exhaust gas, of the catalyst and optionally of the engine. The known method is, however, not suitable for regenerating a storage catalyst for an internal-combustion engine for driving vehicles because it is absolutely necessary to carry along a reducing agent supply in the vehicle.
DE 195 43 219 C1 (corresponding to U.S. Pat. No. 5,771,686) describes a method for driving a diesel engine which uses additional fuel as the reducing agent for the desorption of the storage catalyst. For regenerating the storage catalyst, the internal-combustion engine is therefore operated with a rich mixture (fuel/air ratio .lambda.&lt;1) below the stoichiometric mixture ratio .lambda.=1. The known method provides a rich/lean control of the diesel engine as a function of its operating parameters load, rotational speed and injection quantity. The regeneration of the NO.sub.x storage catalyst is to be implemented not only by a simple lambda jump, but additional measures are suggested which are adapted to the diesel engine in order to achieve the after-engine nitrogen oxide reduction of the diesel engine.
The indicated measures are an exhaust gas recirculation optimized with respect to the exhaust gas aftertreatment system, an adapted intake air throttling and an additional injection of fuel. These measures are intended to make available, on one hand, sufficient reducing agent in the form of hydrocarbons for the NO.sub.x reduction, and, on the other hand, to generate in the exhaust gas for a short time an atmosphere which has a reducing effect to permit the regeneration process. The nitrogen oxide content of the exhaust gas is sensed by an NO.sub.x sensor such that, when an NO.sub.x storage threshold value, which varies with respect to the characteristic diagram as a function of the rotational speed and the load, is reached, a change-over takes place from an operation of the diesel engine with a lambda value &gt;1 to an operation with a lambda value &lt;1. If, when the NO.sub.x threshold value is reached, the NO.sub.x sensor switches to the regeneration operation, in the known method, the exhaust gas composition is to be changed. In addition to the measures for the exhaust gas recirculation, the intake air throttling and the additional afterinjection of diesel fuel are to take place by the optimization of the exhaust gas composition by measuring the soot particles as well as by a control of the regeneration temperature of the storage catalyst.
The known automatic engine control measures are specifically adapted to the operating performance and the requirements of the diesel internal-combustion engine. Furthermore, DE 195 43 219 C1 contains no information as to in which manner the mutually influencing control measures and operating parameters of the internal-combustion engine are to be coordinated when the operating mode is changed. Although the known method indicates a possibility as to how the regeneration conditions of the NO.sub.x storage catalyst can be produced within the scope of the lean/rich control of the diesel engine, it does not address the specific requirements of the mixture preparation when changing the operating mode (stratified charge/homogeneous mixture formation) of a direct-injection Otto cycle internal-combustion engine.